Solutions containing boric acid and/or borates are mainly produced during the running of pressurized water nuclear power stations. Because these solutions are radioactive, solidification treatment is necessary to change them into chemically and physically stable solid bodies in order to ensure nuclear power safety. For solidification treatment of these radioactive solutions, currently the frequently used mainly are the three methods of cement solidification, plastic solidification and bitumen solidification. Among the three methods, the cement solidification has the lowest solidification volume efficiency and as a result, although its operation is of the simplest and the cement solidified bodies are generally considered to possess a long-term safety, however, because the expenses for final disposal of solidified bodies of the radioactive wastes are counted by volume, the cement solidification process will gradually be replaced in a state when expenses for final disposal increase day by day. Both plastic solidification and bitumen solidification methods, on the other hand, use organic materials as the solidification agents. Although higher volume efficiency may be obtained by the two methods, the bitumen solidified bodies are burnable and have a low strength, there have also been instances of burning during the process of operation of bitumen solidification in foreign countries. Many countries in Europe have already banned use of bitumen solidification process and in many other countries except for those bitumen solidification systems that have been established earlier and still continue to be in use and for export to relatively under-developed countries, there is almost no newly built system to join in. The fact that the bitumen solidification process is being gradually eliminated is almost certain. As for the plastic solidification process, its use remains still a subject of dispute; even though newly built systems are continuously joining in, people, who hold a negative point of view, consider that the plastics is a material susceptible to ageing and since the history of use of plastics by mankind lasts merely about 50 years, it has not been possible to ascertain that the quality of plastic solidified bodies of the wastes remain stable for more than 300 years and would not change in substance and therefore, in many countries in Europe the plastic solidification process is no longer used. Generally speaking, the future of use of the plastic solidification process is mainly linked to whether the volume efficiency in solidification of the inorganic solidification agent can be raised to allow the ultimate handling expense to be lowered to an acceptable level. Otherwise, at the pressure of an ultimate handling cost, it may be expected that the plastic solidification process will continue to be adopted because of its excellent volume efficiency in solidification. Speaking from the current situation, research on enhancing the solidification volume efficiency of the inorganic solidification agent so that on the already available basis that the quality of inorganic solidified body is able to ensure a long-term stability, the reduction in volume of the solidified body enables the inorganic solidification method to also possess an advantage on volume efficiency, is a major direction on the current researches on solidification of low radioactive wastes.
The conventional cement solidification technique is also a kind of inorganic solidification method. When the method is used in solidification of borate wastes, generally boric acid is regulated to be alkaline by using sodium hydroxide and after boric acid is concentrated to a solution containing 21,000 ppm, lime and cement are added into it and the solution is sufficiently mixed and is then left motionless to allow it to be solidified. Because there is presence of an impediment effect on cement hydration hardening by boric acid, the content of borate wastes to be added into the cement slurry must not be excessive. Also, the content of boric acid in the borate wastes solidified body produced by the unimproved conventional solidification method is generally suitably not exceeding 5 wt %, if not, there will be problem as to the grade. Adding of lime is an improvement in relation to the conventional cement solidification method, which causes boric acid to form insoluble calcium borate crystals, avoids it from impeding hydration hardening action of the cement and thus helps in enhancing volume efficiency of the solidification. Such a conception has exactly been used in the so-called advanced cement solidification process developed by the Japanese firm, JGC Corporation, in which lime is first added into the liquid borate wastes and the solution at 40-60.degree. C. is agitated for about 10 hr. to allow calcium borate to age and grow crystal. The solution is next filtered to obtain calcium borate crystals and finally, the calcium borate crystals are solidified with cement. By this process, it is said that 190 gal of liquid borate waste containing 21,000 ppm of boron can be solidified into a 55 gal barrel of solidified body. Compared with the conventional method, the volume efficiency of solidification shows a significant improvement, the operation is however tedious and the process is slightly complicated, while the equipment investment is also relatively high.
There still remain many methods for solidification of borate wastes performed with inorganic solidification agents, for instance, in U.S. Pat. No. 4,293,437 or French Patent FR-A-2,423,035, the borate solution is neutralized with alkalizer barite (baryta) having precipitation effect to form a concentrated suspension slurry containing barium borate precipitate. After further adding alkaline silicate acting as a suspension agent, finally cement and bitumen emulsion are again added into the suspension slurry to solidify the slurry. In this process, the boron content is increased by menas of production of a suspending liquid of barium borate precipitate and is finally solidified with cement and bitumen emulsion. It is said that the final solidified product of the process contains 233 g/l of the borate equivalents and the solidification volume efficiency has a higher solidification volume efficiency than the conventional cement solidification process.
In the process disclosed in U.S. Pat. No. 4,210,619, lime is added into the solution containing 11% boric acid and after boric acid is converted into insoluble calcium borate, cement is next added into the slurry obtained and mixed for solidification. In U.S. Pat. No. 4,800,042, lime is also added into the borate solution to convert boric acid into calcium borate and in a further step after calcium borate is filtered and separated it is solidified with cement to obtain a higher solidification volume efficiency than U.S. Pat. No. 4,210,619. The principle of this process is entirely the same as the advanced cement solidification process of the Japanese JGC.
Next, in the U.S. Pat. No. 4,620,947, magnesium oxide or magnesium hydroxide powder is first added into the borate solution to form magnesium borate, into which cement is then added and the mixture is agitated. Finally, before colloids are formed calcium oxide or calcium hydroxide is added for solidification. Following the conditions used in this patent, the concentration of boric acid in the liquid waste is about 10 wt % and weight of the lime, cement, magnesium hydroxide and calcium oxide added is several times the weight of the boric acid. Hence, the volume efficiency is very low and the compressive strength of the solidified bodies produced is also very low, the highest reaching only 22.5 kg/cm.sup.2.
On the other hand, U.S. Pat. No. 4,664,895 discloses a process for solidification of the liquid borate waste by adding sodium metasilicate into a high concentrated borate solution. The boric acid concentration used in this process reaches as high as over 30 wt % of the liquid waste and the process is thus capable of obtaining a relatively high volume efficiency. The compressive strength of its solidified bodies however, lies between 500 psi to 700 psi (35 to 49 kg/cm.sup.2) only, which is not high enough. Most importantly, the solidified product generated in this process is in the state of silicic acid and the water resistance property is not satisfactory.
U.S. Pat. No. 4,906,408 discloses a process for solidification of liquid borate waste and waste resin containing boric acid and according to the process the emphasis is on converting boric acid into calcium boroettringite and calcium monoboroaluminate to avoid any unsatisfactory reaction occurring between borate and cement or water that leads to expansion and cracking in the solidified bodies. According to this process, borate solution with a very low concentration is used and also into each unit volume of the borate solution, 1.75 time volume of the cement and silicon additive must be added. Hence, one can well imagine that the solidification volume efficiency according to this process is also very low.
In the above-mentioned prior art, a majority has adopted a technique of adding alkaline precipitating agent in converting borates into insoluble borides and then adding the solidifying agent, cement or bitumen for solidification, for example, the addition of alkaline barite to cause formation of a suspending liquid of barium borate precipitate in U.S. Pat. No. 4,293,437; addition of lime to cause conversion of borate into insoluble calcium borate in U.S. Pat. Nos. 4,210,619, 4,800,042 and 4,906,408; addition of magnesium oxide or magnesium hydroxide to form magnesium borate in U.S. Pat. No. 4,620,947. In these ways although there is improvement in solidification volume efficiency of the liquid borate waste, from the point of view of the present invention however, such solidification ways are unable to produce adequately the volume efficiency of solidification of boric acid, the reasons being that: (1) the alkaline precipitating agent added has basically increased the amount of the wastes and (2) borates are still regarded as wastes needed to be embedded, the weight percentage of borates within the solidified body is therefore subject to considerable limitation and the solidification volume efficiency can not be enhanced greatly.